An energy storage device like a lithium-ion secondary battery had been used as a power source of mobile equipment such as a notebook computer or a mobile phone. The energy storage device has been recently applied to wider fields and has been used as a power source of an electric vehicle. An energy storage device needs to have long life performance particularly if it is used as a power source of an electric vehicle. Such an energy storage device is expected to be secondarily used as a power source for load leveling after use as a power source of an electric vehicle.
It is accordingly necessary to find a deteriorating state of an energy storage device, and there has been conventionally proposed a technique for accurate detection of a state of an energy storage device (see Patent Document 1, for example). This technique achieves detection of a deteriorating state of a secondary battery according to the property that capacity of a secondary battery linearly decreases relatively to a difference value of the battery capacity, or the property that internal resistance of a secondary battery linearly increases relatively to a difference value of a battery voltage.